The present invention relates to an ophthalmic lens comprising a diffractive part.
Furthermore it relates to a method for designing said ophthalmic lenses.
A wavefront passing the eye will be influenced by the optical parts of the eye such that for example chromatic aberration is provided to the wavefront. The reason is that the refractive indices of the materials in the optical parts of the eye differ for different wavelengths. Thus light having different wavelengths will be refracted a different amount and they will fall on the retina at different places, i.e. different colors can not be focused to the same point. This is called chromatic aberration.
Recently there has been much interest in the correction of the monochromatic aberrations of the eye. It has been revealed that when all monochromatic aberrations are corrected in the human visual system, it serves to unmask the chromatic aberration of the eye. Therefore, in order to optimize the optical quality of the eye, a combination of monochromatic and chromatic aberrations needs to be corrected. A diffractive pattern could be configured to provide a passing wavefront with chromatic aberration of the opposite sign as chromatic aberration from the eye. Thus a diffractive pattern can be used to correct for chromatic aberration introduced to a wavefront from the optical parts of the eye. Some background theory of chromatic aberration can be found in, for example Chapter 17 in xe2x80x9cOptics of the Human Eyexe2x80x9d written by David A. Atchison and George Smith. A theoretical background of the diffractive pattern could be found in the article xe2x80x9cPractical design of a bifocal hologram contact lens or intraocular lensxe2x80x9d, Allen L. Cohen, Applied Optics 31(19)(1992). Ophthalmic lenses, which on at least one surface comprises a diffractive pattern for correcting for chromatic aberration are known from for example U.S. Pat. Nos. 5,895,422, 5,117,306 and 5,895,422. These lenses do, however not, compensate for other aberrations provided by the eye surfaces. In SE 0001925-7, and WO 01/89424, aspheric lenses are designed to compensate for spherical aberration. In some applications these lenses will provide the eye with an increase in chromatic aberration. It is therefor a need of an ophthalmic lens for correcting refractive errors that also can correct for monochromatic and chromatic aberrations.
An object of the present invention is to improve the visual quality for a patient.
A further object of the present invention is to provide an ophthalmic lens, which corrects for chromatic aberration and at least one type of monochromatic aberration.
A further object of the present invention is to provide an ophthalmic lens, which corrects for both chromatic and spherical aberration.
Still a further object of the invention is to correct for spherical aberration as expressed by the 11th normalized Zernike term.
A yet further object is to provide an aspheric lens capable of correcting for spherical aberration having a diffractive part adding refractive power to the lens and providing compensation for chromatic aberration introduced by the optical surfaces of the eye and by the aspheric lens surface. In this text the term aspheric will refer to rotationally symmetric, asymmetric and/or irregular surfaces, i.e. all surfaces differing from a sphere.
These objects are achieved by an ophthalmic lens as initially described in xe2x80x9ctechnical field of inventionxe2x80x9d, which according to the invention further comprises a refractive part comprising at least one surface, which is configured to compensate a passing wavefront at least partly for at least one type of monochromatic aberration introduced by at least one of the optical parts of the eye. The diffractive part is according to the invention capable of compensating a passing wavefront at least partly for chromatic aberration introduced by at least one of the optical parts of the eye. Said refractive and diffractive parts together contribute to a required power of the lens. In this text xe2x80x9cthe optical parts of the eyexe2x80x9d refer to the parts of the eye that contribute to the refraction of incoming light. The cornea of the eye and the natural or an implanted lens are optical parts of the eye. But also inhomogeneities, e.g. in the vitreous are considered as the optical parts of the eye. An optical element that combines both diffractive and refractive optics is called a hybrid element. The monochromatic aberration could be for example astigmatism, coma, spherical aberration, trifoil, tetrafoil or higher aberration terms.
Hereby an ophthalmic lens is achieved that is capable of compensating for at least one type of monochromatic aberration and for chromatic aberration introduced by the optical parts of the eye to a passing wavefront.
Preferably the diffractive part also is capable of compensating a passing wavefront at least partly for chromatic aberration introduced by the refractive part of the lens.
In one embodiment of the invention the monochromatic aberration corrected for is spherical aberration.
The longitudinal chromatic aberration of the eye is very well understood and has been shown to have very similar values from subject to subject (Thibos et. al., xe2x80x9cThe chromatic eye: a new reduced heye model of ocular chromatic aberration in humansxe2x80x9d, Applied Optic, 31, 3594-3600, (1992)). It has also been shown to be stable with age (Mordi et. al., xe2x80x9cInfluence of age on chromatic aberration of the human eyexe2x80x9d, Amer. J. Optom. Physiol. Opt., 62, 864-869 (1985)). Hereby an ophthalmic lens to correct for the average chromatic aberration of the eye could be designed.
Diffractive surfaces can be characterised by their so called phase functions. This phase function describes the additional phase that is added to a ray when it passes the diffractive surface. This additional phase is dependent on the radius of the lens where the ray strikes the surface. For radially symmetric diffractive surfaces this function can be described using Equation 1.                               φ          ⁡                      (            r            )                          =                                            2              ⁢              π                        λ                    ⁢                      (                          DFO              +              DF1r              +                              DF2r                2                            +                              DF3r                3                            +                              DF4r                4                            +              …                        ⁢                          xe2x80x83                        )                                              (        1        )            
Where r is the radial coordinate, xcex the wavelength and DF0, DF1 etc. are the coefficients of the polynomial.
The diffractive part of the lens can also introduce some spherical aberration to a passing wavefront. Preferably, according to the present invention, the refractive part is made capable to compensate a passing wavefront for the spherical aberration introduced by the diffractive part of the lens. Hereby, the spherical aberration could be reduced to a minimum after the wavefront has passed the optical parts of the eye and said lens.
To compensate for the spherical aberration, an aspherical surface, with a lateral height described by Equation 2, could be introduced to the refractive part of the lens. An aspheric surface can be configured to counteract the spherical aberration introduced by the optical parts of the eye and by the diffractive part of the lens. All the optical parts of the eye do not necessarily have to be considered. In one embodiment it is sufficient to measure the spherical aberration introduced by the cornea of the eye and compensate for only the spherical aberration provided by the cornea and optionally also for the spherical aberration introduced by the diffractive part of the lens. For example Zernike terms could be used to describe the optical surfaces of the eye and thus also be used to configure the aspheric surface of the lens, which is adapted to compensate for the spherical aberration. Table 1 shows the first 15 normalized Zernike terms and the aberrations each term signifies. The spherical aberration is the 11th normalized Zernike term. The designing of a lens that is adapted to compensate for aberrations as expressed in Zernike terms is explained in further detail in the Swedish patent application SE 0000614-4 to which is given reference.                     z        =                                                            (                                  1                  R                                )                            *                              r                2                                                    1              +                                                1                  -                                                                                    (                                                  1                          R                                                )                                            2                                        ⁢                                          (                                              cc                        +                        1                                            )                                        ⁢                                          r                      2                                                                                                    +                      ADr            4                    +                      AEr            6                                              (        2        )            
Where R is the radal coordinate of the lens, cc is the conic constant, and AD and AE are coefficients of the polynomial extension.
The spherical aberration of the lens is influenced by the shape factor of the lens. The spherical aberration of a spherical refractive lens can be minimized by a convex-plano lens (Atchison D. A., xe2x80x9cOptical Design of Intraocular lenses. I: On-axis Performancexe2x80x9d, Optometry and Vision Science, 66 (8), 492-506, (1989)). In the present invention, the amount of correction of spherical aberration depends on the shape factor of the lens. It is also possible to use a diffractive pattern that is able to correct for spherical aberration as well as for chromatic aberration. This can be done by modifying the higher orders of the phase function of the diffractive profile (lower orders, or terms on r2 (Equation 1), describe the paraxial properties of the lens).
Other types of monochromatic aberrations can also be corrected for by aspheric refractive surfaces. The shape of the surface becomes more complex the higher the order of the aberration that is corrected. To compensate for a general aberration with an aspherical surface, the lateral height could be described by Equation 3, though also other descriptions are possible.   z  =            ∑              i        =        1            n        ⁢          xe2x80x83        ⁢          z      i      xe2x80x83zi=(asi)xiyk
i=xc2xd[(j+k)2+j+3k]
Where asi are the coefficients of the polynomial.
Preferably the ophthalmic lens together with the eye provides a polychromatic image quality, which when expressed as MTF(50) (Modulation Transfer Function at 50 cycles per millimeter) performs at least about 40% higher than an aspheric lens compensating for the same spherical aberration as the inventive lens but without compensating for the chromatic aberration. A high value of the polychromatic image quality indicates that the amount of chromatic aberration is small and also that the amount of monochromatic aberrations is small.
The lens can correct for the spherical aberrations and the chromatic aberrations as defined in a model eye. Spherical aberration of the eye can run between zero and 1.5 diopter, while chromatic aberration typically runs up to 2.5 diopters (xe2x80x9cOptics of the Human Eyexe2x80x9d written by David A. Atchison and George Smith).
Suitably, the diffractive part is a diffractive surface profile. Such a diffractive surface profile consists of a number of concentric rings. The distances between the rings are decreasing out from the center of the lens. The area between two rings is called a zone. The width of the first zone is a constant that defines the widths of all the other zones. For more background techniques see the article by Allen L. Cohen referred to on page 1 in this application.
In one embodiment, the profile height is equal to one design wavelength. 550 nm is often used as the design wavelength since this is the wavelength for which the retina has its maximum sensitivity. When the profile height is equal to one design wavelength the lens will have its maximum effect in its first order. The profile height is, in another embodiment equal to two design wavelengths and then the lens will have its maximum effect in its second order. See e.g. the aforementioned article by Allen L. Cohen and the U.S. Pat. Nos. 5,895,422, 5,117,306, 5,895,422. The profile height could be any integer number of the design wavelengths.
In one embodiment of the invention the anterior surface of the lens is an aspheric surface, on which a diffractive profile is superimposed. In another embodiment of the invention the anterior surface of the lens is an aspheric surface and the posterior surface of the lens is flat and has a diffractive profile. Also other combinations are possible. For example a diffractive profile could be provided on both the anterior and the posterior surface. Both the anterior and posterior surfaces could also be aspheric. The skilled person can readily identify alternative lens configurations which will be suitable to design the inventive chromatic and monochromatic aberration reducing lenses.
The objects are also achieved by a method as initially described comprising combining a refractive part and a diffractive part of the lens such that they together compensate a passing wavefront at least partly for at least one type of monochromatic aberration and for chromatic aberration introduced by at least one of the optical parts of the eye, while dimensioning said refractive and diffractive parts to provide the lens with a required power.
In one embodiment the method further comprises measuring at least one type of monochromatic aberration provided to a wavefront from at least one of the optical parts of an eye and combining the refractive and diffractive parts of the lens such that they compensate at least partly for the measured monochromatic aberration.
In one embodiment of the invention the measured monochromatic aberration is spherical aberration.
The spherical aberration of the whole eye could be measured using a wavefront sensor. If only the cornea is considered well-known topographical measurement methods could be used. Such topographical methods are disclosed in for example xe2x80x9cCorneal wave aberration from videokeratography: accuracy and limitations of the procedurexe2x80x9d, Antonio Guirao and Pablo Artal, J. Opt. Soc. Am. Opt. Image Sci. Vis., June, 17(6), 955-965, (2000). A wavefront sensor is described in U.S. Pat. No. 5,777,719 (Williams et. al.).
Suitably, the method further comprises measuring the chromatic aberration provided to a wavefront from at least one of the optical parts of the eye and combining the refractive and diffractive parts of the lens such that they together compensate a passing wavefront at least partly for the measured chromatic aberration introduced by at least one of the optical parts of the eye. The chromatic aberration of the eye could be measured by using vernier methods such as those similar to the methods outlined in Thibos et. al., xe2x80x9cTheory and measurement of ocular chromatic aberrationxe2x80x9d, Vision Res., 30, 33-49 (1990) and Marcos et. al, Vision Research, 39, 4309-4323, (1999). Alternative ways for measuring chromatic aberration are described in a textbook, xe2x80x9cOptics of the Human Eyexe2x80x9d by David A. Atchison and George Smith, published by Butterworth-Heinemann, ISBN 0-7506-3775-7.
Preferably, the method further comprises measuring the refractive error of the eye and dimensioning the refractive and diffractive parts of the lens such that they together compensate at least partly for the refractive error of the eye.
With this method of designing an ophthalmic lens the chromatic aberration, the spherical aberration and the refractive error of the eye, could all be considered and compensated for. The lens is designed with one refractive part and one diffractive part and they are combined, such that they together compensate a passing wavefront for these aberrations introduced by the optical parts of the eye.
The aberration corrections could all be full corrections or partial corrections. Furthermore all the corrections could be based on the aberrations of one or more parts of the eye. The corrections could also be based on either an average value of a certain population or on the measured values of the individual patient or on a combination of an average value and individual measurements. The certain population can be a group of people in a specific age interval or for example a group of people having had an eye disease or a corneal surgery. For chromatic aberration the values are pretty much the same for all humans so it is possible to take an average value of all kinds of people and correct for this chromatic aberration in the lens. Of course it is possible to do the same for spherical aberration but in this case it would be preferred to choose a group of people or even measure the spherical aberration for every individual since the spherical aberration will differ more from eye to eye than chromatic aberration.
The ophthalmic lens could be configured to be a phakic or pseudophakic intraocular lens (IOL), a spectacle lens or a contact lens. In the examples described below the lenses are pseudophakic IOLs. The material used in the example lenses described below is a foldable silicone high refractive index material described in U.S. Pat. No. 5,444,106. Other materials are however also possible for these lenses. For example PMMA (Poly-methylmethacrylaat) and hydrogels are suitable materials. The example lenses have a power of 20D. However, the lenses could be designed to have any other suitable power. Also negative lenses are possible.
A method of designing the ophthalmic lens described above comprises the steps of:
i) selecting an eye model with a refractive aspheric ophthalmic lens of a predetermined refractive power and a predetermined amount of at least one monochromatic aberration;
ii) estimating the power of said eye model at different wavelengths, so as to determine the chromatic aberration of the eye model;
iii) estimating a correction function of how the power varies with the wavelength to be an ideal compensation for said chromatic aberration of the eye model;
iv) finding a linear function of how power varies with the wavelength, which suitably approximates said correction function;
v) calculating a provisional zone width of a diffractive profile corresponding to this linear function and also calculating the diffractive power of this diffractive profile;
vi) reducing the refractive power of the refractive ophthalmic lens by the amount of power calculated for the diffractive profile;
vii) estimating a new correction function of step iii), finding a new linear function of step iv) and calculating a new provisional zone width and a new diffractive power for a new diffractive profile corresponding to this new linear function;
viii) adjusting the refractive power of the refractive ophthalmic lens such that the total power of a hybrid lens, which comprises both the refractive ophthalmic lens and the diffractive profile and which is adapted to replace the refractive ophthalmic lens in the eye model, equals the predetermined power;
ix) repeating steps vii) to viii) until a suitable combination of a refractive and a diffractive part of the hybrid ophthalmic lens is found that both provide the eye model with a predetermined power and with a suitable reduction in chromatic aberration.
Suitably this method comprises as a last step measuring the monochromatic aberration of the combination of the eye and the hybrid ophthalmic lens of the method above and correcting the refractive part of the ophthalmic lens according to the measurements such that the monochromatic aberration is reduced sufficiently for the combination of eye and ophthalmic lens.
One example of an eye model that can be used is the eye model of Navarro but other models are also possible. The eye model could also be an individual eye of an individual patient.
In one embodiment the at least one monochromatic aberration of the refractive ophthalmic lens is spherical aberration.
There are different possibilities for the design of the lenses according to the invention. One possibility is to design each lens for each individual. Then the chromatic aberration, the spherical aberration and the refractive error of the eye of the patient are measured and a lens is designed from these values according to the above described method. Another possibility is to use average values from selected categories of people to design lenses adapted to suit almost all the people belonging to this category. It would then be possible to design lenses having different powers but providing the same reduction of spherical and chromatic aberration to patients within these groups of people. The groups of people could for example be age groups or groups of people having had specific eye diseases or a group of people having had a corneal surgery. Furthermore it would be possible to provide a kit of lenses having an average value of chromatic aberration and a range of different values of spherical aberration for each power. This could be preferred since the chromatic aberration is about the same in most human eyes. Hereby it would be necessary to measure the refractive error and the spherical aberration of each individual eye and then choose one lens from this kit of lenses to comply with these measurements.
The following examples are just given as examples and are not intended to be limiting for the invention in any way.